1. Field of the Invention
This invention concerns fluorescent lamps, which are electric lamps comprising a glass envelope having a coating of phosphor on its inner surface, which have electrodes at each end and which contain a fill including low pressure mercury vapor.
This invention particularly relates to protective coatings applied to conventional fluorescent lamp glass envelopes.
2. Description of the Prior Art
It is well known in the art that the light output and the lumen maintenance of fluorescent lamps are affected by a progressive darkening of the bulb during the useful life of the lamp. The darkening is commonly attributed to disclorations resulting from the amalgamation of mercury with sodium at the inner surface of the glass under the influence of impinging ultraviolet radiations. Mercury is present in the lamp fill and sodium is present in the glass.
In a special type of fluorescent lamp the inner surface of the glass is coated with a transparent electroconductive layer of tin oxide or indium oxide in order to achieve satisfactory ignition characteristics. In this particular case, the darkening is compounded by additional discolorations resulting from the conductive coating. This is particularly true of the white conductive SnO.sub.2 which can be reduced to the black SnO.
Many types of protective coatings in fluorescent lamps have been disclosed most of which are relatively thick and porous due to the method of application by dispersion in an organic binder followed by the conventional coating, drying and baking process.
Such is the case for a coating of zinc oxide, titanium oxide or cerium oxide disclosed in U.S. Pat. No. 2,774,903, issued to L. Burns, Dec. 18, 1956.
In U.S. Pat. No. 3,141,990, issued to J. G. Ray, July 12, 1964, the TiO.sub.2 is 12 to 25 microns thick.
Another thick layer of TiO.sub.2 is disclosed in U.S. Pat. No. 3,379,917, issued Apr. 23, 1968 to R. Menelly and an alumina layer of 1 to 10 microns thick with a thin layer of titania is disclosed in U.S. Pat. No. 3,599,029, issued Aug. 10, 1971 to Martyny.
While the thick protective layers achieve some improvement in maintenance, they also introduce the disadvantage of reducing the initial light output.
In order to reduce the initial light loss, much thinner coatings of TiO.sub.2 and ZrO.sub.2 have been disclosed, as in U.S. Pat. No. 3,377,494 to R. W. Repsher on Apr. 9, 1968.
However, thin films of TiO.sub.2 have been noted to cause starting problems in fluorescent lamps, hence the disclosure of Sb.sub.2 O.sub.3 additions to such films in U.S. Pat. No. 3,541,376, issued Nov. 17, 1970 to Sadoski and Schreurs.
In U.S. Pat. No. 3,748,518, issued June 14, 1972 to D. Lewis, it is stated that a titania film 10 to 20 nanometers thick reflects the ultra violet radiation back into the phosphor. U.S. Pat. No. 3,624,444, issued to F. Berthold on Nov. 30, 1971 discloses the necessity of protective layers over tin oxide conductive films to prevent the formation of black stains already occurring after 50 operating hours. In this case, the protective layers have a thickness of 50 to 150 nanometers and consist of oxides of titanium, zirconium, hafnium, niobium and tantalum.
Very thin films with a thickness less than 200 nanometers have so far only been produced by vapor deposition or from hydrolyzed solutions of relatively expensive metal organic compounds such as tetraisopropyl titanate or titrabutyl titanate and require elaborate controls to produce the required thickness with reproducible accuracy.